Stratigraphic controls on the morpho-tectonic evolution of the Nile Gorge, Ethiopia

Abstract

This work aimed at understanding the lithological controls on the SW-flowing segment of the Blue Nile and its ~ 1500-m deep Nile Gorge in the NW Ethiopian Plateau. The gorge exposes a ~ 1100-m-thick Mesozoic sedimentary section overlain by ~ 400-m-thick Cenozoic flood basalt. This work found no significant controls of the river channel by local structure. A NW–SE geologic cross-section was constructed to measure the valley width, the normalized valley width, and the valley asymmetry as a function of depth at 20-m intervals. Additionally, the amount of incision through time was calculated at 1-Ma interval starting 30 Ma. The Blue Nile, during its incision on the NW Plateau, (1) maintains an increase in width of ~ 30 m per 1- m, except when it incised into the upper part of flood basalts (~ 50 m per 1-m depth), a lower part of the Mesozoic sedimentary section dominated by shale–mudstone and sandstone unit (~ 70 m per 1-m depth), and a lowermost unit of the sedimentary section dominated by sandstone unit (10 m per 1- m depth); (2) maintains a NW asymmetry except in the lowermost sandstone unit where it becomes either symmetrical or asymmetrical to the NW or SE; (3) incised for ~ 700- m through the flood basalt and the upper sedimentary section between 30 and 10 Ma, an additional ~ 200- m through the middle sedimentary section between 10 and 6 Ma, and an additional ~ 600- m through the lower sedimentary section between 6 Ma and present.